This invention relates to a digital transmission line test code generator which may be more particularly described as a generator for generating a quasirandom sequence of four-level signals.
Error rate measurements are made on digital transmission lines to provide information indicating the quality of transmission such as excellent, good, fair, poor, and failed. Error rate can be determined by checking parity on an in-service basis or by an out-of-service test using a pseudorandom code sequence. Error rate measurements typically are made on an end-to-end basis because the in-service test requires recovery of framing at the location where the measurement is being made and because the out-of-service test requires external test sets. Recovery of framing is prohibitively expensive at each repeater along the transmission line.
Separate fault location circuits and equipment are being provided for locating suspected or known faults along a transmission line. Typically an individually assigned supervisory signal frequency is uniquely associated with each repeater location for purposes of polling the repeaters for fault location testing. By polling the repeaters with the uniquely assigned frequencies, it is possible to determine where a failure is located.
There are problems resulting from these testing arrangements. When the repeaters with the separate circuits are polled, it is possible to locate a complete failure in the system, but the location of a soft failure producing degraded operation is very difficult to determine. On the other hand, the known error rate monitoring arrangements, which test from end-to-end, can readily determine that a soft failure is affecting operation, but those known arrangements cannot readily determine where the soft failure is located.
It is desirable to develop an error rate measurement system which operates on a section-by-section basis for determining the location of any failure. A pseudorandom code sequence can be used for initiating remote error testing on regenerators along a digital transmission line. To use such a pseudorandom code sequence for testing raises problems because the detector that would be required at every repeater location is complex and because errors appear to multiply as they go through these detectors.